Vehicle steady state handling & weight transfer

[boileroo]

Hi all

I am doing a university module on vehicle dynamics and wish to clarify/enhance my understanding of vehicle handling and the effects of weight transfer. Here are a points based some past exam questions and my answers, if anyone can spot a mistake or would like to add anything this would be appreciated!

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: Increasing the mass at the front.

increasing the mass at the front will move the centre of gravity towards the front of the vehicle and increase the load placed onto the front axle. This will result in an increased weight transfer (WT) at the front of the vehicle and therefore increase front lateral acceleration (LA) and decreased rear lateral acceleration. This increase in front LA will cause an increase in the front tyre slip angle relative to the rear, which will reduce the oversteer, possibly causing mild oversteer.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: an increase in acceleration through an increase in engine torque

An increase in engine torque will cause the COG to move towards the rear of the vehicle, causing an increase on the load of the rear axle and a decrease at the front axle. This will cause 2 things: firstly, the WT at the rear will be increased, causing a larger LA at the rear of the vehicle, and vice versa at the front. This will result in an increase in oversteer.
Secondly, as the effective weight over the front axle is reduced the front tyres will experience less grip while the car is accelerating. This will mean that the front tyres' slip angle will increase. Under heavy acceleration the majority of the tyres grip will be used on forward motion, and so less grip is available for cornering, which can result in understeer.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: changing the old worn tyres for new.

if the old tyres are changed then the tyres will have relatively more grip available to them, so for any given level of acceleration there will be more grip available for turning and the car will tend to oversteer more. NB feel as though I'm missing something here, doesn't feel like enough for the marks available!

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: changing the front suspension geometry so that the tyres lean into the bend.


Changing the geometry such that the front wheels lean into a corner will result in an increase in oversteer as the tyres will generate an increased force into the bend, increasing the body roll of the vehicle and therefore increasing the weight transfer. The camber will also increase the effective grip of the tyre due to the contact patch area being increased, also causing an increase in oversteer

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: moving the drive to the rear wheels

this would result in moving the COG towards the rear of the vehicle, which will cause an increase in the rear weight transfer (and reduction in the front). Also, the effective load over the front axle will be reduced, and therefore the grip of the front tyres will suffer. These 2 factors will contribute to changing the steady state handling characteristic to understeer, especially at high levels of acceleration.


Q: A front wheel drive car is set up for mild oversteer, explain the effects of: an aero wing increasing downforce at the rear

The rear wing will increase the load on the rear axle, increasing the grip available to the rear tyres. This will cause a decrease in the rear tyre slip angle, and so will reduce the oversteer experienced by the driver. This effect will only be noticeable at speed, as the force produced by the wing is proportional to speed, so at low speed the effect will be negligible.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: reducing the front suspension damping

unsure, some help would be appreciated!


Many thanks for any responses!

 

[Ranger Mike]

Q: A front wheel drive car is set up for mild overseer, explain the effects of: Increasing the mass at the front.

increasing the mass at the front will move the center of gravity towards the front of the vehicle and increase the load placed onto the front axle. This will result in an increased weight transfer (WT) at the front of the vehicle and therefore increase front lateral acceleration (LA) and decreased rear lateral acceleration. This increase in front LA will cause an increase in the front tyre slip angle relative to the rear, which will reduce the oversteer, possibly causing mild oversteer.

Manufacturers will design a FWD. car to under steer or over steer ..it depends on its intent. That said..if you have a car set up to be loose...what you don’t want to do is add more front end weight nor make any more weight transfer to the front than practical. Adding Mass to the front may not increase LA because you may add to the slip angle...loss of traction. The COG does not move. A percentage of Weight is transferred to the front and to the side ( depending upon the corner) when entering a turn. This weight is tranfered thru the Roll Center to the tire contact patch.


Q: A front wheel drive car is set up for mild overseer, explain the effects of: an increase in acceleration through an increase in engine torque

An increase in engine torque will cause the COG to move towards the rear of the vehicle, causing an increase on the load of the rear axle and a decrease at the front axle. This will cause 2 things: firstly, the WT at the rear will be increased, causing a larger LA at the rear of the vehicle, and vice versa at the front. This will result in an increase in oversteer.
Secondly, as the effective weight over the front axle is reduced the front tyres will experience less grip while the car is accelerating. This will mean that the front tyres' slip angle will increase. Under heavy acceleration the majority of the tyres grip will be used on forward motion, and so less grip is available for cornering, which can result in understeer.

Under acceleration, a percentage of weight will shift to the rear. Let me clarify the COG does move In the purest sense but for any noticeable displacement we are talking about raising the automobile many inches to shift the COG slightly. In fact that is how we calculate the COG of a custom built race car..using wheel scales and jacking the car at least 14 “ to note a shift. You are correct on the loss of traction and thus pushing in the turn under acceleration.



Q: A front wheel drive car is set up for mild oversteer, explain the effects of: changing the old worn tyres for new.

if the old tyres are changed then the tyres will have relatively more grip available to them, so for any given level of acceleration there will be more grip available for turning and the car will tend to oversteer more. NB feel as though I'm missing something here, doesn't feel like enough for the marks available!

Ifin the car was set up to oversteer, then adding new tires will make it oversteer...old tires will not have as much traction and hence, will push..or understeer

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: changing the front suspension geometry so that the tyres lean into the bend.


Changing the geometry such that the front wheels lean into a corner will result in an increase in oversteer as the tyres will generate an increased force into the bend, increasing the body roll of the vehicle and therefore increasing the weight transfer. The camber will also increase the effective grip of the tyre due to the contact patch area being increased, also causing an increase in oversteer

Proper camber build is critical to maintaining the best tire contact patch. I assume this is what the question is about. Also Ackermann and proper Toe Out and caster are in the mix.



Q: A front wheel drive car is set up for mild oversteer, explain the effects of: moving the drive to the rear wheels

this would result in moving the COG towards the rear of the vehicle, which will cause an increase in the rear weight transfer (and reduction in the front). Also, the effective load over the front axle will be reduced, and therefore the grip of the front tyres will suffer. These 2 factors will contribute to changing the steady state handling characteristic to understeer, especially at high levels of acceleration.

This is a loaded question. Rear engine rear drive cars tend to understeer.. front engine rear wheel drive cars are notoriously loose..over steer because the damn things are too nose heavy.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: an aero wing increasing downforce at the rear

The rear wing will increase the load on the rear axle, increasing the grip available to the rear tyres. This will cause a decrease in the rear tyre slip angle, and so will reduce the oversteer experienced by the driver. This effect will only be noticeable at speed, as the force produced by the wing is proportional to speed, so at low speed the effect will be negligible.

any time your moving, you are moving air...but aero wings disfussers, spoilers, splitters are good over 60 MPH and really come on over 100 MPH..and why down force only at the rear?

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: reducing the front suspension damping

soft shocks up front will increase understeer..tie down shocks on the back will tighten up a car ..reduce oversteer

good job..welcome to the forum

 

[Kozy]

I'll just answer them as I would.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: Increasing the mass at the front.

The mild oversteer will be generally be engineered through a higher rear roll stiffness, which increases the load differential at the rear axle and decreases rear grip as the lateral acceleration increases. That is to say, under steady state cornering, as the lateral acceleration rises, so does oversteer.

With an increase in front weight distribution, the front tyres gain more grip through higher vertical load, but also have to hold more lateral load through the increased mass. Generally, the two effects will cancel out. However, with an increase of front load transfer, the rate at which the oversteer increases with lateral G will reduce, resulting in a net increase of understeer at all accelerations.

In addition, with less mass at the rear, the rear axle will reach 100% LT at an earlier point resulting in inside rear wheel lift. From this point, the understeer increases with further lateral acceleration. If IRWL is encountered, understeer may increase dramatically at higher lateral accelerations.

Despite an increase in steady state understeer, with more front %, lift off oversteer will increase. With the same proportion of forwards weight transfer, the heavier front end will see a bigger differential of front/rear load and as a result, a bigger differential in grip. This is because the vertical load on the rear tyres is reduced, however the lateral load remains the same since the weight is unchanged.

Considering both points, a FWD with a high front % can display a bigger differential between steady state understeer and lift off oversteer, and would generally be considered more difficult to drive.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: an increase in acceleration through an increase in engine torque

Slip angle effects aside, an increase in forwards acceleration will increase load over the rear wheels and reduce it at the front. Rear axle grip is improved with more load vertical load, yet without an actual increase in static weight, the lateral load remains constant. Net result is an increase in understeer.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: changing the old worn tyres for new.

Assuming this to mean a full set of new tyres, offering a higher co-efficient of friction, then the lateral acceleration limit will be increased, and with the trend towards increasing oversteer with rising lateral acceleration, oversteer will also increase.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: changing the front suspension geometry so that the tyres lean into the bend.


Increasing front camber will increase front end grip up to a point. More oversteer.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: moving the drive to the rear wheels

Too many variables.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: an aero wing increasing downforce at the rear

Addition of aerodynamic load has the effect of increasing tyre grip through vertical load without an additional lateral loading. With more rear grip as a result, the car will understeer.

Q: A front wheel drive car is set up for mild oversteer, explain the effects of: reducing the front suspension damping

Reducing front damping will increase oversteer during transitions as it will slow the rate of load transfer at the front end. A slower rate of LT means the car takes longer to react to inputs, so although grip is improved, response may suffer.

 

[boileroo]

thank you very much for your thoughts, very useful!

Thanks

 

[Kozy]

It should be noted, on that last one about damping, that if we are looking purely at steady state cornering, that the dampers will have no effect.

Dampers require motion to create force, and at full roll the suspension has displacement but no motion.

 

[mender]

1. Adding weight to the front of the car increases the load on the front tires. This results in lower lateral acceleration capability at the front because of load sensitivity of tires.
http://en.wikipedia.org/wiki/Tire_load_sensitivity

2. In the case of mild oversteer, mild acceleration will result in a reduction of the oversteer from weight transfer alone depending on the C of G height. If the acceleration is excessive, the tires will not be able to sustain both the cornering forces and acceleration forces and excessive understeer will result.
http://en.wikipedia.org/wiki/Circle_of_forces

5. With no other changes (component locations), adding drive to the rear tires will decrease oversteer to a point due to the weight transfer as in #2. However, more than mild acceleration will see a transition back to oversteer.

7. None. If the vehicle is cornering at steady state, changing the damping will have no effect as the dampers only control transient movement. Just to confirm Kozy's post!

 

[mender]

Generally, the two effects will cancel out.

No; see tire load sensitivity graph.

If they canceled out we wouldn't have to worry about weight transfer or weight distribution or roll couple distribution or ...